The Depths of Human Consciousness – Dr. Sharath Krishnaswami

His craft plunged into the water, his stance turning from supine with his knees flexed, to a sitting position, once the cylindrical submersible dived headlong into the deep. He quickly oriented himself to the change in position and took charge of the controls. Months of dedicated training in yoga conditioned his body to facing work in cramped conditions, not to mention claustrophobia.

At the surface, it would be a warm 30ºC to a cold 5ºC as he moved deeper into the water. His electronically heated vacuum packed clothing would make him work comfortably as the water grew colder, almost freezer like once he reached the bottom. Around him would be oxygen tanks air scrubbers, wires, switches and screens. He would use joysticks to control the sub’s thrusters, and manoeuvre the craft while watching the feed from the external camera booms. 1

He would spend the next two hours, descending deeper at the rate of 5 knots (1 knot = 1.85 km/hr), his destination, the Mariana Trench, the deepest point of the Earth. He would communicate to the ship on the surface, the ‘Mermaid Sapphire” through an acoustic modem, that would shift the frequency of his voice and send powerful bursts of voice through the water. If voice messaging failed, he would switch to text messaging. A daunting thought, considering he would soon reach the deepest point of the Mariana Trench, the Challenger Deep, 11,000 metres from sea level. 1

As James Cameron reached 4 km below the surface, the Abyssal Zone, it was pitch dark. The cameras caught creatures glowing in the depths, abyssal crustaceans and molluscs2, vampire squids and telescope octopi. His craft would constantly send information about the sub, the temperature, oxygen and carbon dioxide levels as well as the levels of energy remaining in the batteries that powered the sub.

At around 100 metres from the bottom, Cameron would prepare the sub for landing. He would release steel pellets to lighten the sub and slow its descent. Once the sub lands softly at the ocean floor, 8 metre long LED lights would power on, and he would look for sights no human had ever seen before. 1

“I see the Deep Sea Challenger is a tribute to the engineering, that’s been sound and solid for over 30 years or more of deep ocean exploration, and some fairly radical new ideas. The pressure sphere itself, the design of the hatch, and the design of the acrylic viewport, are all based in proven designs. We had to our own detailed execution of it, and it took a lot of work to find that element analysis”, he says in an interview with Ron Allum 3, an Australian submarine designer, cave diver and inventor. James Cameron most notable for being the director of the film Titanic, is more importantly, a diver who made 33 trips to the bottom of the Atlantic to view the sunken ship. 5

The sheer scale of his expeditions and projects are quite prohibitive for us, but it is akin to what we do as Neuroanaesthesiologists. If the patient is the craft, we submerse the patients to deeper levels of consciousness, so that the Neurosurgeon can work in a plane which gives him optimum conditions to make sure no untoward outcome is expected. However, the window for disaster is great and like James Cameron mentions “There is no excuse for what happened here” 5 regarding the implosion of the Titan submersible which hoped to explore the ruins of the Titanic 6

And yet, similarities between the amount of care afforded to the processes of exploring the depths of the ocean, and immersing the human to the depths of unconsciousness are uncannily similar.

The Neurosurgeon uses a high level of manual dexterity to be able to perform tasks like tying surgical knots or placing aneurysm clips. They pay a high attention to detail and need to have excellent coordination and should be able to perform specific procedures with precision. 7

“If a neurosurgeon makes a mistake, the damage is often permanent.” Henry Marsh, one of Britain’s foremost neurosurgeons, a senior consultant at St. George’s Hospital, in London says in an interview with The New Yorker. 8

And to facilitate this process, is what we as Neuroanaesthesiologists strive towards. Achieving optimal operating conditions for the neurosurgeon while ensuring that the patient maintains unconsciousness, good brain blood flow and oxygen levels is vital for us as perioperative physicians.9 The host of the measures by which we monitor the patients consciousness can be represented by intraoperative brain mapping (cortical and subcortical mapping) during surgeries that involve lesions near  the functional or ‘eloquent’ cortex and intraoperative EEG and bispectral index (BIS) which help to effectively guide the induction of burst suppression during clipping of aneurysms. 10

The study of human consciousness has evolved over the many years that led us to the present methods of measurement during surgery. John Snow, in 1847, described five degrees of narcotism for ether anaesthesia. 11

“In the first degree” he says, “includes any effects of chloroform that exists while the patient possesses perfect consciousness of where he is, and what is occurring around him.  It is designated as a kind of inebriation, which is usually agreeable when induced for curiosity, but is often otherwise, when the patient is about to undergo an operation. This stage is very transitory. Although it is the property of narcotic vapours to suspend the functions of different parts of the nervous system in succession, yet they probably influence every part of that system from the first, but in different degrees. Common sensibility becomes also impaired, so that the pain of disease, which is generally due to a morbid increase of the common sensibility, is in many cases removed, or relieved, according to its intensity.” 12

His description was further refined by Artusio, Guedel, and Woodbridge who defined Anaesthesia as having four components, sensory blockade, motor blockade, blockade of autonomic reflexes and loss of consciousness. Prys-Roberts further found a common feature of general anaesthesia is suppression of conscious perception of noxious stimuli. 11

I, however, feel the explanation of consciousness in the Neuroanaesthesia and Neurosurgical textbooks lacking. Boris Kotchoubey 13 in his article published in the section of theoretical and philosophical psychology (Journal: Frontiers of Psychology) discusses that consciousness, is not a process in the brain but a kind of behavior that is controlled by the brain. Human consciousness was postulated to emerge on the interface between three components of animal behavior: communication, play, and the use of tools. Dennet(1991) et al, regarded it as a kind of information processing. Searle, 2000 and Beckermann, 2005 described “human awareness” as the ability to experience one’s own “internal states”.  Brentano, 1982, further expanded on the understanding as internal states that are “about” some external objects.  13

I believe, that to dim the response of the body to these external objects, is what we as neuroanaesthesiologists employ to prime the patient for critical neurosurgeries.

Notwithstanding the devices used to measure consciousness, we have however, hit a paradox in its measurement. Researches in the Picower Institute for Learning and Memory highlight that is hard to measure a system without the measurement affecting the system. Researchers assessing consciousness, using volunteers to receive anesthesia, typically use spoken commands to see if subjects can still respond, but that sound might keep them awake longer or wake them up sooner than normal.

“We want to measure when people make the transition from conscious to unconscious, and vice versa, but as soon as you ask someone to do something, which is the classic way of assessing this, you’ve now influenced them and disrupted the process,” said Christian Guay, lead author of the study in the British Journal of Anaesthesia. 14

Which begs the question, how else can we measure consciousness? The more recent studies, for example, by Casali 15, provides a quantitative metric, a numerical measure of consciousness, that separates the vegetative state from a minimally conscious state.

In Casali’s case, the neural activity is evoked directly in the brain using a transient magnetic field (Transcranial Magnetic Stimulation). This involves applying a transient magnetic field, which generates an electric field in a particular region of the brain due to Faraday’s law, a bit like attaching a battery to the neural circuitry. This causes currents to flow in the brain, not just in the stimulated region, but in other regions connected to it as well. The spatial and temporal patterns of these currents in the brain are then inferred from the EEG measurements and quantified to produce the metric. 16

A more recent study by Wanchun Wu et al 17, aimed to study whether olfactory response could be a sign of consciousness and represent higher cognitive processing in patients with disorders of consciousness (DoC) using clinical and electroencephalogram data. Their study stemmed from the notion by Mori et al., (2013). Merrick et al. (2014) who believed that the olfactory system could be used to distinguish between conscious and unconscious processing since it has its own phenomenological, cognitive, and neurodynamic properties apart from its anatomical characteristics.  Nigri et al, (2016) found that central olfactory processing was reported to show various degrees of preservation in patients with DoC and had a clear relationship with their consciousness. 28 patients with disorders of consciousness were recruited into the study. The researchers investigated the presence of an olfactory response in these patients and divided them into two groups: ORES group (the presence of olfactory response) and N-ORES group (the absence of olfactory response). They then collected the olfactory electroencephalogram (EEG) data from each patient along with data on healthy controls. The olfactory response was assessed using vanillin (pleasant odor), decanoic acid (unpleasant odor), and a blank. An olfactory response was present in 16 out of 28 patients (57%), and their analysis found out that the presence of an olfactory response in patients with DoC was associated with a higher level of consciousness further indicating that olfactory responses could help diagnose the consciousness of patients with DoC.

Well, if we’ve gone so deep in suspending awareness, it’s only apt that we figure ways to help the patients achieve consciousness faster after general anaesthesia. But that is left for another day and another article, the details of which our Neurosurgeon colleagues will be only too happy to discover and to speed up the process of recovery. For now, all we can be concerned about is how James Cameron returned to the surface and got back to land.

While he explored the bottom of the Challenger Deep using a joystick to power the 12 ton vehicle, looking for new animal species and sampling rocks, he sucked up new creatures and collected rocks for scientists eagerly waiting at the “Mermaid Sapphire”. After spending 6 hours in the sub, he flipped a switch and dropped more than 450 kilograms of iron ballast weights onto the ocean floor. The foam beam of the sub helped rise it to the surface of the ocean, after being freed from the forces of gravity. A floatation bag would soon emerge from a hatch, and get filled with Nitrogen gas, popping his sub up to the surface so that mankind can find him. 1


  10. Khandelwal A, Jangra K, Katikar MD, Durga P, Maheswara Rao GSU. Choosing Neuroanaesthesia as a career: Marching towards new horizons. Indian J Anaesth. 2021 Jan;65(1):35-42. doi: 10.4103/ija.IJA_1531_20. Epub 2021 Jan 20. PMID: 33767501; PMCID: PMC7980245.
  11. Prof. H.L. Kaul, Dr. Neerja Bharti; MONITORING DEPTH OF ANAESTHESIA. Indian J. Anaesth. 2002; 46 (4) : 323-332
  12. The John Snow, Archive and Research Companion;
  13. Kotchoubey, B. (2018) Human consciousness: Where is it from and what is it for, Frontiers. Available at: (Accessed: 03 July 2023).
  14. Neuroscience News (2023) Studying consciousness without affecting it, Neuroscience News. Available at: (Accessed: 03 July 2023).
  15. A theoretically based index of consciousness independent of … – science. Available at: (Accessed: 03 July 2023).
  16. Mitra, P. (2014) A new method to measure consciousness discovered, Scientific American. Available at: (Accessed: 03 July 2023).
  17. Wu, W. et al. (2023) Olfactory response is a potential sign of consciousness: Electroencephalogram findings, Frontiers. Available at: (Accessed: 03 July 2023).

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